JPH0773657B2 - Wet Flue Gas Desulfurization Method - Google Patents

Description

TECHNICAL FIELD The present invention relates to an improvement in a wet flue gas desulfurization method, and particularly to a wet flue gas desulfurization method using an alkaline compound such as magnesium hydroxide, caustic soda, slaked lime or lime as an absorbent. .

[Conventional technology]

At present, the flue gas desulfurization method is mainly used, and an absorbing solution containing an alkaline compound such as magnesium hydroxide or calcium carbonate as an absorbent is used to bring the flue gas into gas-liquid contact with the sulfur content of the flue gas to form a sulfate salt. There is a so-called wet flue gas desulfurization method.

For example, the following chemical reactions generally occur in a cooling tower and an absorption tower in a wet flue gas desulfurization apparatus using magnesium hydroxide [Mg (OH) ₂ ] as an absorbent.

Explaining each reaction formula here, Eq. (1) is a reaction that absorbs sulfur dioxide in exhaust gas into the circulating fluid, and Eq. (2) is sulfur dioxide (H ₂ SO ₃ ) generated in Eq. (1) and circulating fluid. The reaction of magnesium sulfite (MgSO ₃ ) in the solution, the formula (3) is the reaction when magnesium hydroxide (Mg (OH) ₂ ) is injected into the circulating fluid, and the formula (4) is that the magnesium sulfite in the circulating fluid is oxygen. Shows the reaction of being oxidized by.

The following two points are important in order to carry out the above series of reactions continuously.

(1) First, it is necessary to prevent the generation of hardly soluble magnesium sulfite. If this crystal is deposited, there is a possibility that troubles such as scaling in the cooling tower and absorption tower and blockage of pipes and valves may occur. Since magnesium sulfate and magnesium dithionate (Mg (HSO ₃ ) ₂ ) have high solubility, there is no risk of trouble due to crystal precipitation.

(2) Next, it is important that magnesium sulfite and magnesium dithionite are present in appropriate amounts in the circulating liquid in order to smoothly proceed the reactions of the above formulas (2) and (3).

Generally, when the pH of the circulating fluid (hydrogen ion concentration) is around 6, the concentration of magnesium sulfite and magnesium hydrogen sulfite is considered to be 0.03 to 0.1 mol / liter in terms of sulfite ion. Here, an example of a conventional process flow around a cooling tower and an absorption tower of a wet flue gas desulfurization apparatus using magnesium hydroxide as an absorbent will be described with reference to FIG.

The exhaust gas 101 is a cooling tower 104 attached to the absorption tower 106.
It is cooled by the circulating liquid that is introduced into the spray nozzle 105 and sprayed from the spray nozzle 105. At this time, the exhaust gas is usually cooled from 100 to 200 ° C to 50 to 70 ° C. Simultaneously with cooling, the cooling tower 104 collects soot and dust in the exhaust gas and absorbs part of the sulfurous acid gas. The cooled exhaust gas 102 is introduced into the intermediate portion of the absorption tower 106 and passes through the inside of the packed bed 107 filled in the central portion of the absorption tower that is wetted by the circulating liquid sprayed from the spray nozzle 108 at the upper part of the absorption tower. To rise. Gas-liquid contact is performed in this process, and the sulfurous acid gas in the exhaust gas is absorbed by the circulating liquid. Exhaust gas that has passed through the packed bed 107 has its carrier overmist removed by a demister 109 near the discharge port in the upper part of the absorption tower and is discharged as a processing gas 103.

On the other hand, the circulating liquid flowing down through the cooling tower 104 and the absorption tower 106 is temporarily accumulated in the liquid chamber 110 below the absorption tower 106. A line 111 connecting the side wall of the liquid chamber 110 and the circulation pump 112 and the circulation pump 11
Line 113 and line 1 connecting 2 to the spray nozzle 108
An absorbing liquid circulation system is formed by 15 and the circulating liquid in the liquid chamber 110 flows through this circulation system. Further, via a line 114 branched from the line 113, the spray nozzle 10 of the cooling tower 104
Part of the absorption liquid is circulated to 5.

The water required for the circulatory system is the line 116 on the side wall of the absorption tower 106.
More usually, it is supplied so as to keep the level of the liquid chamber 110 within a certain range.

Further, necessary magnesium hydroxide is supplied from a line 117 on the side wall of the absorption tower 106. The supply amount is usually the pH of the circulating fluid.
Is adjusted to a predetermined value. The magnesium hydroxide is usually supplied as a slurry having a concentration of 5 to 30%.

The dust and reaction products accumulated in the circulation system are blown from a line 118 branched from the line 113 and sent to a wastewater treatment device (not shown).

The oxidation reaction in the equation (4) proceeds by oxygen contained in the exhaust gas in the cooling tower 104 and the absorption tower 106, but the oxygen concentration in the exhaust gas is usually 1 to 6% (in the case of boiler exhaust gas). Since it is low, the reaction does not proceed sufficiently. Therefore, the air taken in from the line 119 having the air intake is pressurized by the blower 120 and then connected to the trachea 122 in the liquid chamber of the absorption tower 106.
It is supplied to the liquid chamber 110 via 121 to cause the oxidation reaction of the above formula (4). Due to this aeration, the circulating liquid in the liquid chamber 110 is agitated to prevent the accumulation of soot and dust, and
The mixing of magnesium hydroxide supplied from 117 into the circulating liquid and the promotion of the desulfurization reaction are promoted.

[Problems to be solved by the invention]

 The known prior art has the following drawbacks.

(1) When a large amount of air is supplied into the liquid chamber in the lower part of the absorption tower, the circulation pump sucks in air, causing problems such as cavitation.

(2) If the air supply to the liquid chamber in the lower part of the absorption tower is stopped due to a blower failure, etc., the trachea is likely to become clogged, and if clogging occurs, the normal aeration will not occur even if the air supply is restarted. I can't.

(3) Since the circulating liquid in the absorption tower is usually controlled to around pH 6 and Mg (OH) ₂ is supplied to it, the reactivity is poor.

[Means for solving problems]

The present invention relates to (1) a wet flue gas desulfurization method in which an absorbent containing an alkaline compound as an absorbent is circulated in an absorption tower while being contacted with an exhaust gas containing a sulfur compound, and a part of the absorbent-containing circulating liquid is ejected. Air is sucked in and the sulfite ions generated in the desulfurization reaction are oxidized to sulfate ions, then transferred to a gas-liquid separator, where the separated liquid is jetted into the circulating liquid in the lower liquid chamber of the absorption tower. A wet flue gas desulfurization method, and (2) a wet flue gas desulfurization method in which an absorbent containing magnesium hydroxide as an absorbent is circulated in an absorption tower and brought into contact with exhaust gas containing a sulfur compound, A part of the absorbent-containing circulating liquid is supplied to the ejector to suck air, oxidize sulfite ions generated by the desulfurization reaction to sulfate ions, and then transfer them to a gas-liquid separator where they are separated. By replenishing the liquid with magnesium hydroxide and jetting it into the circulating liquid in the lower liquid chamber of the absorption tower, it is possible to adjust the concentration and pH of magnesium sulfite and magnesium hydrogen sulfite in the absorbing liquid circulating in the absorption tower. It is a characteristic wet flue gas desulfurization method.

〔Example〕

 An embodiment of the present invention will be described below with reference to FIG.

The exhaust gas 4 is introduced into a cooling tower 4 attached to the absorption tower 6, and is cooled by a circulating liquid containing a magnesium hydroxide absorbent sprayed from a spray nozzle 5 in an exhaust gas flow passage portion in the cooling tower 4. At this time, the exhaust gas is usually from 100 to 200 ° C
Cooled to 50-70 ° C. Simultaneously with cooling, the cooling tower 4 collects dust and particles in the exhaust gas and absorbs part of the sulfurous acid gas.

The cooled exhaust gas 2 is introduced into the middle part of the absorption tower 6 and rises up through the inside of the packed bed 7 in the central part of the absorption tower, which is wetted by the circulating liquid sprayed from the spray nozzle 8 in the upper part of the absorption tower 6. Gas-liquid contact is performed in this process, and the sulfurous acid gas in the exhaust gas is absorbed by the circulating liquid. The exhaust gas that has passed through the packed bed 7 has its carrier mist removed by a demister 9 near the discharge port in the upper part of the absorption tower and is discharged as the processing gas 3.

On the other hand, the circulating liquid flowing down through the cooling tower 4 and the absorption tower 6 is temporarily accumulated in the liquid chamber 10 below the absorption tower 6. A line 11 that connects the side wall of the liquid chamber 10 and the circulation pump 12 and a line 13 and line 15 that connects the circulation pump 12 and the spray nozzle 8 form an absorption liquid circulation system, and the circulation liquid in the liquid chamber 10 is circulated. Flowing through the system. Further, the absorption liquid is partially circulated to the spray nozzle 5 of the cooling tower 4 via the line 14 separated from the line 13.

The water required in this circulation system is normally supplied from a line 16 on the side wall of the absorption tower 6 so as to keep the level of the liquid chamber 10 within a certain range.

The required magnesium hydroxide is the line on the side wall of the absorption tower 6.
Supplied from 17. The supply amount is usually adjusted so that the pH of the circulating liquid becomes a predetermined value. The magnesium hydroxide is usually supplied as a slurry having a concentration of 5 to 30%.

The dust and reaction products accumulated in the circulation system are blown from a line 18 branched from the line 13 and sent to a wastewater treatment device (not shown).

The oxidation reaction in the equation (4) proceeds by oxygen contained in the exhaust gas in the cooling tower 4 and the absorption tower 6, but the oxygen concentration in the exhaust gas is usually 1 to 6% (in the case of boiler exhaust gas, ) Is low, the reaction does not proceed sufficiently. Therefore, a configuration is adopted in which a part of the circulating liquid is introduced into the ejector 20 via the line 21 branched from the line 13. A line 19 for sucking air is attached to the edger 20. Also, this editor
The gas 20 is connected to the gas-liquid separator 23 by a line 22, and the air flowing from the line 19 and the circulating liquid are in gas-liquid contact with each other inside the ejector 20 and the line 22, and the oxidation reaction of the sulfite ion generated by the desulfurization reaction is performed. Is done. Then, the air introduced into the gas-liquid separator 23 and the circulating flow are separated here. Since the circulating liquid in the gas-liquid separator 23 undergoes the following chemical reaction, it usually has a low pH.

Mg (HSO ₃ ) ₂ + O ₂ → MgSO ₄ + H ₂ SO ₄ (5) Then, magnesium hydroxide is supplied from the line 26 connected to the gas-liquid separation 23, so that the conventional absorption tower liquid chamber 110 shown in FIG. The reactivity is better than that in the case of supplying to.

Next, the circulating liquid separated in the lower part of the gas-liquid separator 23 is jetted into the absorption tower liquid chamber 10 through the circulating liquid return line 24 to stir the inside of the liquid chamber.

The air separated at the top of the gas-liquid separator 23 is
Through the upper part of the absorption tower liquid chamber 10.

In the above-mentioned embodiment, the case where magnesium hydroxide is used as the alkali absorbent has been described, but it is natural that it can be applied when the alkaline agent such as caustic soda, slaked lime or limestone is used and the oxidation of sulfite ion is necessary.

〔The invention's effect〕

As described above, according to the present invention, the air for sulfite ion oxidation is carried out by an ejector, and only the liquid part after the gas-water separation is jetted and supplied to the absorption tower liquid reservoir, so to the circulation pump of the absorption tower circulation system. The air will not be sucked in and the cavitation can be prevented.

At the same time, by adopting the above configuration, it is not necessary to install an air supply device having a drive unit that is likely to cause a failure such as a blower, and it is not necessary to install a trachea in the absorption tower liquid chamber. There is no fear that the circulation liquid cannot be agitated by.

Further, since the gas portion after gas-water separation is returned to the absorption tower liquid chamber, it is possible to prevent soot dust from accumulating in the liquid chamber by stirring the circulating liquid.

Further, since the alkaline absorbent can be replenished to the steam-water separator having a low pH immediately after oxidizing the sulfite ion, there is an effect that the reactivity is improved.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of a wet flue gas desulfurization apparatus of the present invention, and FIG. 2 is a block diagram of a conventional wet flue gas desulfurization apparatus.

Claims (2)

[Claims] 1. A wet flue gas desulfurization method in which an absorbent containing an alkaline compound as an absorbent is circulated in an absorption tower while being brought into contact with exhaust gas containing a sulfur compound, and a part of the absorbent-containing circulating liquid is supplied to an ejector. To suck air,
Wet flue gas, characterized in that after sulfite ions produced in the desulfurization reaction are oxidized to sulfate ions, they are transferred to a gas-liquid separator and the liquid separated there is jetted into the circulating liquid in the lower liquid chamber of the absorption tower. Desulfurization method. 2. A wet flue gas desulfurization method in which an absorbent containing magnesium hydroxide as an absorbent is circulated in an absorption tower while being brought into contact with an exhaust gas containing a sulfur compound, and a part of the absorbent-containing circulating liquid is supplied to an ejector. After supplying and sucking air, sulfite ions generated in the desulfurization reaction are oxidized to sulfate ions and then transferred to a gas-liquid separator, where the separated liquid is supplemented with magnesium hydroxide to lower the liquid chamber of the absorption tower. A wet flue gas desulfurization method, characterized in that the concentration and the pH of magnesium sulfite and magnesium hydrogen sulfite in the absorption liquid circulating in the absorption tower are adjusted by injecting it into the circulation liquid.